Introns and Exons Introns and Exons Introns--Introns--

Untranslated Untranslated intervening intervening sequences in mRNAsequences in mRNA

Exons– Translated Exons– Translated sequencessequences

Process-RNA Process-RNA splicingsplicing

Heterogeneous Heterogeneous nuclear RNA nuclear RNA (hnRNA)-Transcript (hnRNA)-Transcript before splicing is before splicing is complete complete

Splicing OverviewSplicing Overview Occurs in the nucleusOccurs in the nucleus hnRNAs complexed with specific proteins, hnRNAs complexed with specific proteins,

form a form a ribonucleoprotein particle (RNP)ribonucleoprotein particle (RNP) Primary transcripts assembled into Primary transcripts assembled into hnRNPhnRNP Splicing occurs on Splicing occurs on spliceosomes spliceosomes consist ofconsist of Small nuclear ribonucleoproteins Small nuclear ribonucleoproteins (SnRNPs)(SnRNPs)

components of spliceosomescomponents of spliceosomes Contain small nuclear RNA (Contain small nuclear RNA (snRNAsnRNA)) Many types of snRNA with different functions Many types of snRNA with different functions

in the splicing processin the splicing process

Spliceosome Spliceosome

Splice Site RecognitionSplice Site Recognition

IntronsIntrons contain invariant 5’-GU and 3’-AG sequences contain invariant 5’-GU and 3’-AG sequences at their borders (GU-AG Rule)at their borders (GU-AG Rule)

Recognized by Recognized by small nuclear ribonucleoprotein particles (snRNPs) that catalyze the cutting and splicing reactions.

Internal intron sequences are highly variable even Internal intron sequences are highly variable even between closely related homologous genes.between closely related homologous genes.

Alternative splicing allows different proteins from a Alternative splicing allows different proteins from a single original transcriptsingle original transcript

Simplified Splicing Simplified Splicing MechanismMechanism

Close-up of Internal A Close-up of Internal A

AlternatiAlternative ve

Splicing ISplicing I Exon Exon

removed removed with intronwith intron

AlternativAlternative Splicing e Splicing

IIII Multiple 3’ Multiple 3’

cleavage cleavage sitessites

EX. AG EX. AG found at found at 5’ end of 5’ end of exon 2 exon 2 and inside and inside exon 2exon 2